Super absorbent polymer (SAP) is a synthetic polymer material capable of absorbing moisture from about 500 to about 1,000 times its own weight, and it has been also called a super absorbency material (SAM), an absorbent gel material (AGM) and so on. Such super absorbent polymers started to be practically applied in sanitary products, and now they are widely used for preparation of various products, for example, hygiene products such as disposable diapers for children, water retaining soil products for gardening, water stop materials for the civil engineering and construction, sheets for raising seedling, fresh-keeping agents for food distribution fields, or the like.
As a method for preparing such a super absorbent polymer, an inverse suspension polymerization method, an aqueous solution polymerization method or the like are known. Among them, the preparation of super absorbent polymers via inverse suspension polymerization is disclosed in Japanese Patent Publication Nos. Sho 56-161408, Sho 57-158209, Sho 57-198714, and so on. Furthermore, for the preparation of super absorbent polymers via aqueous solution polymerization, a thermal polymerization method of polymerizing a hydrous gel phase polymer while breaking and cooling the same in a kneader equipped with a plurality of spindles, and a photo-polymerization method of exposing a high-concentrated aqueous solution to UV rays or the like on a belt so as to carry out the polymerization and drying at the same time are known.
On the other hand, the absorption rate, one of important physical properties of super absorbent polymers is associated with the surface dryness of the product in contact with a skin, such as disposable diapers. In general, these absorption rates can be improved in a manner of widening a surface area of the super absorbent polymer.
As an example, a method of forming a porous structure on the particle surface of the super absorbent polymer by using a blowing agent has been applied. However, general blowing agents have a disadvantage that a sufficient amount of the porous structure cannot be formed and thus the absorption rate is not highly increased.
As another example, there is a method for increasing a surface area of the super absorbent polymer by reassembling fine particles obtained during preparation of the super absorbent polymer to form a porous particle with irregular shape. However, although the absorption rate of the super absorbent polymer can be improved through these methods, there is a limit that a centrifuge retention capacity (CRC) and an absorbency under pressure (AUP) of the polymer are relatively decreased. In this way, because physical properties such as an absorption rate, a centrifuge retention capacity, an absorbency under pressure of the super absorbent polymer have a trade-off relation, there is an urgent need for the preparation method capable of improving these properties simultaneously.